The present invention relates to a referencing system for an array of detector elements of a thermal imager and in particular to a referencing system for use with a thermal imager having a linear array of detector elements over which a viewed scene is scanned.
In most thermal imager systems it is necessary to process the signal received from the output of an array of detector elements of the imager, in order to compensate for DC offset in the output between different elements of the array, and to compensate for differences in responsivity (gain) between the elements. This is referred to as "normalizing" the detector array.
DC offset between elements of a detector array can be compensated for by exposing all the elements to a thermal reference source of uniform temperature. Certain types of detector arrays such as pyro-electric arrays require exposure to a uniform field in order to function, for they rely on the change of a charge which occurs between being exposed to a closed, or shuttered, field and being exposed to the scene to be viewed. The exposure to the shuttered field compensates every frame for any DC offset. With such pyro-electric type detectors, the difference in gain between the elements can be compensated for periodically by exposing the detector elements to a uniformly intense open field, and this may be done once only in the life of the detector during the manufacturing process.
With photovoltaic type detectors there is no requirement to shutter the detector array and therefore there is no inherent DC offset compensation, but normally both DC offset and non-uniformity gain characteristics have to be compensated for. This can be achieved by exposing the detector array to uniform light and dark fields during an initial calibration process, however the DC offset between elements is dependent on the mean temperature of the viewed scene. The gain characteristics of the elements are also unlikely to be linear and therefore any compensation for the difference in gain characteristics will vary in dependence on the mean temperature of the viewed scene. In certain applications it is preferable that differences in offset and gain characteristics of the detector elements of photovoltaic detectors are compensated for in use.
Many photovoltaic imaging systems employ a linear array of detector elements over which the image is scanned. With such a system there will normally be a "flyback" period when the scanning mechanism returns to its starting position. Even with a scanning mechanism employing a multi-faceted polygonal mirror surface where the scanner itself does not have a flyback period, because the final image generated is normally to be a TV-type display which itself has a flyback period, there is a period in which information is not read out from the detector, or the information read out is not utilized in the final display. The "flyback" period can conveniently be utilised to normalize the elements of the array by exposing them to two uniform thermal reference sources at different temperatures.
U.S. Pat. No. 4 948 964 discloses a method for normalizing a photovoltaic detector array. This is achieved by exposing the detector array to a uniform thermal reference source. The detector array is exposed to the reference source by a scanning minor surface which is inserted in the optical path of the detector array during the flyback period. The mirror surface is arranged such that during the flyback period the detector array views a thermo-electric cooler at a uniform temperature. The mirror surface itself forms a radial sector of a disc rotated about an axis, and comprises two portions of different reflectivities. The two portions of different reflectivities enable two reference temperatures to be observed, the highly reflective portion exposing the detector array to a temperature approximately equal to that of the thermo-electric cooler, while the partially reflective portion exposes the detector array to a temperature between the temperature of the thermo-electric cooler and the temperature of the mirror surface itself.
The problems with the above described method are that the scanning mirror surface has to be sufficiently large in order to ensure that the whole detector array is irradicated by the thermo-electric cooler and that it is difficult to control the range of the two reference sources, for the range of the partially reflective mirror surface will depend upon the temperature of the mirror surface itself, which in turn will depend upon the temperature of the working environment. It is an aim of the present invention to provide an improved referencing system for a thermal imager.